Monday, October 24, 2011

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Traditionally the RF specifications have been developed separately for the different 3GPP radio-access technologies GSM/EDGE, UTRA, and E-UTRA (LTE). The rapid evolution of mobile radio and the need to deploy new technologies alongside the legacy deployments has, however, lead to implementation of different Radio-Access Technologies (RAT) at the same sites, often sharing antennas and other parts of the installation. A natural further step is then to also share the base-station equipment between multiple RATs. This requires multi-RAT base stations.

The evolution to multi-RAT base stations is also fostered by the evolution of technology. While multiple RATs have traditionally shared parts of the site installation, such as antennas, feeders, backhaul or power, the advance of both digital baseband and RF technologies enables a much tighter integration. A base station consisting of two separate implementations of both baseband and RF, together with a passive combiner/splitter before the antenna, could in theory be considered a multi-RAT base station. 3GPP has, however, made a narrower, but more forward-looking definition.
In a Multi-Standard Radio (MSR) base station, both the receiver and the transmitter are capable of simultaneously processing multiple carriers of different RATs in common active RF components. The reason for this stricter definition is that the true potential of multi-RAT base stations, and the challenge in terms of implementation complexity, comes from having a common RF.

This principle is shown in the illustration with an example base station capable of both GSM/EDGE and LTE. Much of the GSM/EDGE and LTE baseband functionality may be separate in the base station, but is possibly implemented in the same hardware. The RF must, however, be implemented in the same active components as shown in the illustration.

The main advantages of an MSR base station implementation are twofold:

• Migration between RATs in a deployment, for example from GSM/EDGE to LTE, is possible sing the same base station hardware. In the example shown in the illustration, a migration is performed in three phases using the same MSR base station. In the first phase, the base station is deployed in a network for GSM/EDGE-only operation. In the second phase, the operator migrates part of the spectrum to LTE. The same MSR base station will now operate one LTE carrier, but still supports the legacy GSM/EDGE users in half of the band available. In the third phase, when the GSM/ EDGE users have migrated from the band, the operator can configure the MSR base station to LTE-only operation with double the channel bandwidth.

• A single base station designed as an MSR base station can be deployed in various environments for single-RAT operation for each RAT supported, as well as for multi-RAT operation where that is required by the deployment scenario. This is also in line with the recent technology trends seen in the market, with fewer and more generic base-station designs. Having fewer varieties of base station is an advantage both for the base-station vendor and for the operator, since a single solution can be developed and implemented for a variety of scenarios.

The single-RAT 3GPP radio-access standards, with requirements defined independently per RAT, do not support such migration scenarios with an implementation where common base-station RF hardware is shared between multiple access technologies, and hence a separate set of requirements for multi-standard radio equipment is needed.
An implication of a common RF for multiple RATs is that carriers are no longer received and transmitted independently of each other. For this reason, a common RF specification must be used to specify the MSR base station. 3GPP in release 9 has developed MSR specifications for the core RF requirements and for test requirements. Those specifications support GSM/EDGE,3 UTRA and E-UTRA, and all combinations thereof.

To support all possible RAT combinations, the MSR specifications have many generic requirements applicable regardless of RAT combination, together with specific single-access-technology-specific requirements to secure the integrity of the systems in single-RAT operation.
The MSR concept has a substantial impact for many requirements, while others remain completely unchanged. A fundamental concept introduced for MSR base stations is RF bandwidth, which is defined as the total bandwidth over the set of carriers transmitted and received. Many receiver and transmitter requirements for GSM/EDGE and UTRA are specified relative to the carrier center and for LTE in relation to the channel edges. For an MSR base station, they are instead specified relative to the RF bandwidth edges, in a way similar to carrier aggregation in release 10. In the same way as for carrier aggregation, a parameter Foffset is also introduced to define the location of the RF bandwidth edges relative to the edge carriers. For GSM/EDGE carriers, Foffset is set to 200 kHz, while it is in general half the channel bandwidth for UTRA and E-UTRA. By introducing the RF bandwidth concept and introducing generic limits, the requirements for MSR shift from being carrier centric towards being frequency block centric, thereby embracing technology neutrality by being independent of the access technology or operational mode. While E-UTRA and UTRA carriers have quite similar RF properties in terms of bandwidth and power spectral density, GSM/EDGE carriers are quite different. The operating bands for which MSR base stations are defined are therefore divided into three Band Categories (BC):

• BC1 – All paired bands where UTRA FDD and E-UTRA FDD can be deployed.

• BC2 – All paired bands where in addition to UTRA FDD and E-UTRA FDD, GSM/EDGE can also be deployed.

• BC3 – All unpaired bands where UTRA TDD and E-UTRA TDD can be deployed.

Since the carriers of different RATs are not transmitted and received independently, it is necessary to perform parts of the testing with carriers of multiple RATs being activated. This is done through a set of multi-RAT Test Configuration, specifically tailored to stress transmitter and receiver properties. These test configurations are of particular importance for the unwanted emission requirements for the transmitter and for testing of the receiver susceptibility to interfering signals (blocking, etc.). An advantage of the multi-RAT test configurations is that the RF performance of multiple RATs can be tested simultaneously, thereby avoiding repetition of test cases for each RAT. This is of particular importance for the very time-consuming tests of requirements outside the operating band over the complete frequency range up to 12.75 GHz.
The requirement with the largest impact from MSR is the spectrum mask, or the operating band unwanted emissions requirement, as it is called. The spectrum mask requirement for MSR base stations is applicable for multi-RAT operation where the carriers at the RF bandwidth edges are either GSM/EDGE, UTRA, or E-UTRA carriers of different channel bandwidths.

The mask is generic and applicable to all cases and covers the complete operating band of the base station. There is an exception for the 150 kHz closest to the RF bandwidth edge, where the mask is aligned with the GSM/ EDGE modulation spectrum for the case when a GSM/EDGE carrier or a 1.4/3 MHz E-UTRA carrier is transmitted adjacent to the edge.
An important aspect of MSR is the declaration by the base station vendor of the supported RF bandwidth, power levels, multicarrier capability, etc. All testing is based on the capability of the base station through a declaration of the supported Capability Set (CS), which defines all supported single RATs and multi-RAT combinations.